The present invention relates to an electron source having on an electrically insulating support a first type of parallel electrodes serving as cathode conductors and carrying a plurality of micropoints made from an electron emitting material, a second series of parallel electrodes serving as grids and which are electrically insulated from the cathode conductors and forming an angle therewith, which defines intersection zones between the cathode conductors and the grids, each of the electrodes of one of the series being in contact with a resistive layer and having a lattice structure, thus having tracks which intersect and define first openings, whereby each of the electrodes of the other series is discontinuous and consequently has second openings.
The invention more particularly applies to the display field and more specifically to flat screens.
Micropoint emissive cathode electron sources are already known from the following documents:
(1) French patent application 86 01024 of 24, Jan. 1986 and corresponding to U.S. Pat. No. 4,857,161 , PA1 (2) French patent application 87 15432 of 6, Nov. 1987 and corresponding to U.S. Pat. No. 4,940,916 and PA1 (3) French patent application 90 07347 of 13, Jun. 1990.
In particular, document (3) discloses an electron source of the type referred to hereinbefore and whose electrodes consequently have a lattice structure.
An embodiment of this known electron source is diagrammatically shown in plan view in FIG. 1A and in sectional view in FIG. 1B, which is the section CC of FIG. 1A.
This known source has a matrix structure and comprises an e.g. glass substrate 2 and optionally on the latter a thin silica layer 4. On the latter is formed a series of electrodes in the form of parallel conductive strips serving as cathode conductors and constituting the columns of the matrix
FIGS. 1A and 1B show one of these cathode conductors 5. The cathode conductors are in each case covered by a resistive layer 7. A silica electrically insulating layer 8 covers the resistive layers
Above the insulating layer 8 is formed a series of electrodes, which ape also in the form of parallel conductive strips and whereof one appears in FIGS. 1A and 1B carrying the reference 10. These electrodes formed above the insulating layer 8 are perpendicular to the cathode conductors and serve as grids constituting the rows of the matrix structure.
The known source also has a plurality of micropoints forming elementary electron emitters.
In each of the intersection zones of the cathode conductors and the grids, the resistive layer 7 corresponding to said zone supports micropoints 12 and the grid corresponding to said zone has a hole 14 facing each of the micropoints 12. Each of the latter substantially adopts the shape of a cone, whose base rests on the resistive layer 7 and whose apex is level with the corresponding opening.
Obviously, the insulating layer 8 is also provided with openings 15 permitting the passage of the micropoints 12. Moreover, each of the electrodes of one of the two series of electrodes has a lattice structure in contact with a resistive layer.
In the example shown in FIGS. 1A and 1B, each cathode conductor has said lattice structure and consequently has intersecting conductive tracks SA. Therefore each cathode conductor has openings 6 defined by the said tracks 5A. The micropoints occupy central regions of the lattice meshes.
The electrodes of the other series (the grids in the example shown) have a continuous structure (disregarding the small diameter holes 14 positioned facing the micropoints 12).
The use of electrodes having a lattice structure is aimed at minimizing the breakdown risks at the micropoints, by limiting the electric current therein, so as to prevent the formation of short-circuits between the rows and columns via said micropoints. However, the aforementioned known source whose characteristics have just been described suffers from a disadvantage. Thus, due to the defects which the insulating layer 8 may have, there is a short-circuit possibility in the overlap zones between the lattice electrodes and the continuous electrodes.
On referring to FIG. 1A it can be seen that the surface of the overlap zones is equal to the surface of the lattice structure electrodes.
The object of the present invention is to reduce short-circuit risks more than is possible in the known source and to this end proposes reducing the overlap zones of the two series of electrodes to an even greater extent than in the source known from (3).